Friction question

A 1000kg boat is traveling at 90km/h when its engine is shut off. The magnitude of frictional force fk between boat and water is proportional to the speed v of the boar: fk=70v, where v is in meters per second and fk is in newtons. find the time required for the boat to slow to 45km/h.

The question does not ask for acceleration, so do I need to find the acceleration before finding the time?

In addition, Does any one knows how to use a math template for posting this type of question?